how to market my app for free?

[Jake]

Read about SEO most of the traffic generating methods should work, you may want to create a site for the App or a more general Children's book review site that features yours to attract search ranking and visitors, that links to the store. A free Google Blogger account should do, spending $12 for a domain name may be worthwhile.

You could write articles on animals in trouble, arranging to guest post it with a link on an influential site, or on publishing sites like Ezinearticles..com it's a close call whether to link to your blog site (generally more acceptable) or attempt to link directly to the store.